Search results for "BOND"
showing 10 items of 3527 documents
N‐[tert‐Butoxycarbonylglycyl‐(E)‐α,β‐dehydrophenylalanylglycylglycyl‐(E)‐α,β‐dehydrophenylalanyl]glycine
2006
In the molecule of the title hexapeptide, Boc0–Gly1–ΔEPhe2–Gly3–Gly4–ΔEPhe5–Gly6–OH, C31H36N6O9, there are two overlapping β-turns, one of type II on the ΔEPhe2 (ΔEPhe is isomer E of the α,β-dehydrophenylalanine residue) and Gly3 residues and the second of type III′ on the Gly3 and Gly4 residues. All amino acids in the peptide are linked trans to each other. Three relatively strong intramolecular N—H⋯O hydrogen bonds stabilize the crystal structure. Two of them, of the 4→1 type, are responsible for two β-turns in the peptide.
Conformational investigation of α, β-dehydropeptides
2009
The crystal structure of Ac-Pro-delta Val-NHCH3 was examined to determine the influence of the alpha,beta-dehydrovaline residue on the nature of peptide conformation. The peptide crystallizes from methanol-diethyl ether solution at 4 degrees in needle-shaped form in orthorhombic space group P2(1)2(1)2(1) with a = 11.384(2) A, b = 13.277(2) A, c = 9.942(1) A, V = 1502.7(4) A3, Z = 4, Dm = 1.17 g.cm-3 and Dc = 1.18 g.cm-3. The structure was solved by direct methods using SHELXS-86 and refined to an R value of 0.057 for 1922 observed reflections. The peptide is found to adopt a beta-bend between the type I and the type III conformation with phi 1 = -68.3(4) degrees, psi 1 = -20.1(4) degrees, p…
Coupling of the guanosine glycosidic bond conformation and the ribonucleotide cleavage reaction: implications for barnase catalysis.
2007
To examine the possible relationship of guanine-dependent GpA conformations with ribonucleotide cleavage, two potential of mean force (PMF) calculations were performed in aqueous solution. In the first calculation, the guanosine glycosidic (Gχ) angle was used as the reaction coordinate, and computations were performed on two GpA ionic species: protonated (neutral) or deprotonated (negatively charged) guanosine ribose O2 ′. Similar energetic profiles featuring two minima corresponding to the anti and syn Gχ regions were obtained for both ionic forms. For both forms the anti conformation was more stable than the syn, and barriers of ∼4 kcal/mol were obtained for the anti → syn transition. Str…
2,5-Dialkoxysubstituierte Oligo- und Poly(1,4-phenylenethenylen)e
1994
2,5-Dialkoxy Substituted Oligo- and Poly(1,4-phenyleneethenylene)s O-Alkylation and regioselective Rieche formylation of 2-methylhydroquinone (1) yields the 2,5-dialkoxy-4-methylbenzaldehydes 4a–j. The corresponding azomethines 5a–j enter in a strongly alkaline medium a selfcondensation reaction leading to the title compounds 6/7a–j. These conjugated oligomers and polymers possess highly regular structures with exclusively (E)-configurated double bonds. GPC, IR, 1H-, 13C-NMR and MS-FD measurements were used for their characterization. Somewhat different results were obtained for the similarly prepared Schiff base 5l which contains chlorine substituents in the side chains. Cleavage of hydrog…
ChemInform Abstract: trans- and cis-Bicyclo(n.1.0)alk-2-ynes of Medium Ring Size.
2010
Starting from cycloalkenes, the cis- or trans-cyclopropano anellated cycloalkynes 2–7 of medium ring size were synthesized. The triple bonds were introduced in the final reaction step either by the fragmentation of the corresponding 1,2,3-selenadiazoles or by direct dehydration of a ketone (Scheme 2). The cis-fused bicycles 2 and 3 and the trans-fused bicycles 5, 6 and 7 could be isolated in a pure state. The intermediary existence of the most strained system trans-bicyclo[7.1.0]dec-2-yne (4) was proven by trapping reactions.
trans- andcis-Bicyclo[n.1.0]alk-2-ynes of Medium Ring Size
1997
Starting from cycloalkenes, the cis- or trans-cyclopropano anellated cycloalkynes 2–7 of medium ring size were synthesized. The triple bonds were introduced in the final reaction step either by the fragmentation of the corresponding 1,2,3-selenadiazoles or by direct dehydration of a ketone (Scheme 2). The cis-fused bicycles 2 and 3 and the trans-fused bicycles 5, 6 and 7 could be isolated in a pure state. The intermediary existence of the most strained system trans-bicyclo[7.1.0]dec-2-yne (4) was proven by trapping reactions.
Chemistry with Roataxanes: Intra- and Intermolecularly Covalently Linked Rotaxanes
2006
The direct introduction of sulfonamide units (cf. 9) into carboxamide-based rotaxanes allows us to intramolecularly bridge the “wheel” and the “axle” of such species for the first time as is shown by the bridged bissulfonamide rotaxane 11. Due to its stronger acidity the SO2-NH proton can be selectively abstracted by mild bases even in the presence of CO.NH and then be substituted by treatment with suitable iodo compounds. This leads intramolecularly to 11 (71% yield) and intermolecularly to bis[2]rotaxane 16 (76% yield). The iodo-substituted rotaxane 15 isolated as a remarkably stable byproduct offers a new synthetic potential demonstrated by the preparation of 16.
Molecular “Floppyness” and the Lewis Acidity of Silanes: A Density Functional Theory Study
2001
A comprehensive set of Lewis acid-base adducts of silanes was investigated by means of the density functional theory geometry optimization [B3LYP/6−31G(d)], and thermochemical calculations, [B3LYP/6−311+G(2d,p)//B3LYP/6−31G(d)]. Complex formation was found to weaken Si−Cl and Si−Br bonds more than Si−F or Si−H bonds. Comparable distances between Si and a Lewis base L (L = NH3, OH2, F−) are shorter in hexa- than in pentacoordinated complexes. The molecular structures of the pentacoordinated Si complexes allowed for a mapping of an SN2 reaction pathway by correlating the lengths of the Si−X and Si−L bonds. Complex formation was found to be exothermic for most of the coordination compounds, an…
FT-IR and dielectric study of water/AOT liquid crystals
2000
Abstract In order to explore the influence of microwave radiation on highly viscous microheterogeneous systems, the evolution of structural and dynamical properties of the water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT) liquid crystals as a function of the molar ratio R (R=[water]/[AOT]) has been investigated by FT-IR spectroscopy and time domain reflectometry. The study emphasises how the progressive hydration of the surfactant head groups is mainly responsible for the structural and dynamical evolution of water/AOT liquid crystals. In particular, it has been found that the state of water at lower R values is strongly perturbed, bulk-like water appears only at R>23 and the water/AOT in…
Structural study of NaNdGa4S8, a luminescent material with low-concentration quenching
1988
Abstract Single-crystal X-ray diffraction analysis and optical investigations of NaNdGa 4 S 8 show that Nd 3+ ions are essentially distributed in two of the three square antiprismatic sites of a CaGa 2 S 4 -type lattice. The NdS 8 polyhedra are isolated (minimum NdNd distance: 6.07 A) so the interactions responsible for concentration quenching of the luminescence are considerably reduced